A significant proportion of loudspeakers which are currently commercially available are of the type commonly known as bass reflex loudspeakers. A bass reflex system (also known as a ported, vented box or reflex port) is a type of loudspeaker enclosure that uses the sound from the rear side of the diaphragm to increase the efficiency of the system at low frequencies as compared to a typical closed box loudspeaker or an infinite baffle mounting. Referring to FIG. 1, a bass reflex loudspeaker 1 typically has one or more openings 3 in the loudspeaker enclosure 5 (called reflex ports or vents), each of which usually consists of a rigid pipe, duct or tube 7 (typically circular or rectangular in cross-section) mounted in the front or rear face of the loudspeaker enclosure 5, leading from the air volume 9 behind the driver 11 to the external air. The air 13 in this opening 3 behaves as an acoustic mass whereas the air 9 contained within the enclosure 5 behaves as an acoustic compliance; together these form an acoustic resonator known as a Helmholtz resonator. The frequency at which this acoustic resonance occurs is determined by the length and cross sectional area of air in the opening 3, the volume of air 9 within the enclosure 5, and the speed of sound. Acoustic radiation from the rear of the loudspeaker 1 passes through this acoustic resonator, the acoustic resonator providing in a band-pass response. At the frequency of this acoustic resonance the high pressure in the enclosure 5 reduces the cone motion and acoustic radiation from the front of the loudspeaker diaphragm 15. The combined output from the opening 3 and front of the diaphragm 15 is in the form of a 4th order high-pass filter. With suitable choice of driver parameters, enclosure volume and port dimensions a desired response can be achieved. For example a maximally flat 4th order butterworth high-pass response is the most obvious choice but, depending on the design constraints, there are numerous other choices.
Reflex systems are widely used since they provide a better combination of efficiency and low frequency extension compared to closed box systems. They also have the benefit of reducing the diaphragm excursion at frequencies around the enclosure tuning frequency where the duct provides the main acoustic output.
In use, the low frequency response of a loudspeaker is strongly dependant on room dimensions, construction materials and the relative positions of listener and loudspeaker. Furthermore, what is an acoustically desirable performance is often a subjective choice, with different listeners preferring enhancement or attenuation of different sound frequencies; however, any single size and configuration of reflex port will have a predetermined and largely fixed effect on the overall acoustic performance of a loudspeaker. One practical method of adjusting the low frequency response of a loudspeaker system is to vary the Helmholtz resonance by altering the properties of the port.
For example, to vary the acoustic performance of a reflex-type loudspeaker, cylindrical plugs (bungs) of foamed polyurethane or felt have been inserted into the opening of the reflex port, and these adjust the resonance in the port by partially blocking it; such an approach is rather crude, such “bungs” introduce turbulence and resistive losses with only a minor change in tuning frequency. The resistive losses introduced by air flowing through the porous bung reduce the bass output of the port with no improvement to low frequency extension. The small decrease in tuning frequency does produce a small improvement in low frequency extension. The turbulence occurs where high air velocity gradients occur in the air flow due to small port area, abrupt changes in port area or discontinuities such as those introduced by the bung. The effect of turbulence is to introduce spurious noise and distortion, and also results in a loss of low frequency energy; consequently turbulence may limit undistorted bass output.
A better approach to adjusting the Helmholtz frequency is to provide the user with some means to adjust the port length and/or area. To avoid turbulence the ports should preferably be flared at both ends avoiding discontinuities of the wall surface along the length of the port.
For example interchangeable ports of different dimensions have been suggested in GB2352924. In such arrangements, a range of ducts, each of different length and/or cross-sectional area, is provided, so that a listener can change the acoustic performance simply by removing and replacing one duct with another of different dimensions. Such arrangements necessitate the storage of a number of alternative ducts which, because reflex ports are usually substantially rigid, takes up an undesirable amount of space. In addition, such replaceable ports need to be firmly fixed in position relative to the loudspeaker enclosure, so a user needs to have tools available to be able to undo a duct for removal and to secure a duct in place. Accordingly such systems are not attractive to users.
Another performance limitation of ports is the response peak due to longitudinal acoustic resonance occurring within the duct. This resonance is highly undesirable since it can cause audible colouration. A method of reducing such resonances is described in GB2488758. Providing the user with interchangeable ports constructed in this way would be highly beneficial, but extremely costly.